<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<style>
    /* 实现原理
    1.首先要一个盒子
    2.宽高为0
    3.四个边都让它变圆
    4.四个边 右边设置为透明色
     */
    div {
        width: 0;
        height: 0;
        border-right: 150px solid transparent;
        border-left: 150px solid red;
        border-top: 150px solid red;
        border-bottom: 150px solid red;
        border-top-left-radius: 50%;
        border-bottom-left-radius: 50%;
        border-top-right-radius: 50%;
        border-bottom-right-radius: 50%;
    }
</style>

<body>
    <div></div>
</body>

</html>